Usually, a paint comprises three components, namely a pigment, a binder, and a solvent.
The pigment provides the color, appearance, opaqueness, and certain special properties of the paint, the binder is a macromolecular substance for bonding together the pigments and fillers for keying the paint on the substrate, and the solvent serves to dissolve the binder and to facilitate application of the paint by adjusting the viscosity and the rate of evaporation of said solvent. Once deposited on the substrate that is to be painted, the solvent evaporates and the paint polymerizes or is transformed by chemical bonding.
Unfortunately, the solvents used generally include volatile organic components that are dangerous for humans and the environment. Environmental standards are becoming ever more severe, so it is appropriate to use paints that are not polluting.
It is then possible to envisage using paints in which the solvent is water. Nevertheless, such paints sometimes contain additives, of the glycol ether type, that are harmful for the environment.
Consequently, another type of paint has been implemented, namely powder paints. Powder paints are powdery materials in the form of thermoplastic or thermosetting powders that do not contain any solvents.
The powder paint is then deposited on the substrate for painting and the powder is then transformed into a sheet of paint by a thermal effect, e.g. by baking in an oven. When the substrate for painting is a metal, the person skilled in the art charges the powder paint electrically and then applies it to the substrate for painting. The substrate is electrically charged with a polarity opposite to that of the powder paint, so the paint powder is attracted towards the substrate and adheres thereto temporarily.
Then, by heating the powder paint, e.g. by stoving, the powder paint is caused to melt and then to harden so as to form a sheet of paint covering the substrate.
That method is very effective, but it is unsuitable as it stands for use with electrically non-conductive substrates, i.e. substrates made of thermosetting composite materials, or thermoplastic materials, for example, or indeed plastics type materials.
Furthermore, the method requires the substrate for painting to be held by an end of its outside surface. That method is therefore unsuitable for parts that do not have any surface that can be left unpainted, or parts that are to be completely covered in paint.
In a first method of applying powder paint on an electrically non-conductive material, a conductive material, e.g. graphite, is included in the substrate for painting in order to improve its conductivity.
In a second method, the substrate is covered in an electrically conductive primer layer. The powder paint can then adhere to the substrate via its primer layer.
That method satisfies requirements by enabling non-polluting powder paint to be used on a substrate that is electrically non-conductive.
Nevertheless, the primer layer used may itself possibly be harmful for the environment. In addition, the primer layer imparts an electrically conductive nature to the substrate for painting, and this characteristic may be unacceptable, depending on the use to which the substrate is to be put.
Consequently, a third method is known seeking to remedy that drawback by avoiding giving an electrically conductive nature to a substrate that is to remain electrically neutral.
According to document EP 0 927 082, an electrically conductive primer layer is applied to the substrate for painting, e.g. using a spray gun, and it is then covered in powder paint.
A heating cycle is then performed serving to bake the powder paint. In addition, because of the nature of the primer layer, the painted substrate returns to being non-conductive at the end of the heating cycle.
Although effective, it is found that that third method still requires the use of primer layers.
Furthermore, document FR 2 180 676 presents a method of varnishing a molded part, and document EP 0 698 476 relates to applying a coating referred to as a “gel-coat” on a molded composite sheet. Those documents are thus remote from the invention.